1. Field of the Invention
The present invention relates to a microcomputer capable of interfacing with a peripheral circuit having a signal level different from that of the microcomputer.
2. Description of the Prior Art
Today, semiconductors used especially in communication control applications may sometimes need to address a signal level different from an operating voltage level of the semiconductors. Malfunction is known to occur in some of such contingencies.
FIG. 6 is a block diagram of an output circuit incorporated in a conventional microcomputer. In FIG. 6, reference numeral 10 represents a microcomputer; 11 represents a power terminal to which a supply voltage VDD1 is fed; 12 represents a serial output terminal of the microcomputer 10; 13 represents a step-down circuit for lowering the supply voltage VDD1 coming from the power terminal 11 down to an output voltage VDD2; 14 represents a P-channel MOSFET; and 15 represents an N-channel MOSFET. The gate of the P-channel MOSFET 1 and that of the N-channel MOSFET 15 are supplied with a signal S1 coming from a stage upstream of the output circuit. The serial output terminal 12 outputs a signal T1.
The output circuit of FIG. 6 works as follows. FIG. 7 is a timing chart showing temporal changes of signals included in FIG. 6. In FIG. 7, reference characters VOH represent a lower limit of a High-level signal output from the serial output terminal 12. That is, VOH denotes a lower limit of the specified High level of the signal to a peripheral circuit.
The supply voltage VDD1 entering from the power terminal 11 powers a CPU (not shown) and other parts of the microcomputer 10 and is sent to the step-down circuit 13. The step-down circuit 13 brings the supply voltage VDD1 down to the output voltage VDD2 that is fed to an output stage made up of the P-channel MOSFET 14 and N-channel MOSFET 15. That is, whereas the circuits inside the microcomputer 10 operate on the supply voltage VDD1, the level of the signal output from the serial output terminal 12 falls within the voltage range of the output voltage VDD2 output from the step-down circuit 13.
The supply voltage VDD1 may decrease due to an increase in the load on the microcomputer 10 or because of a lack of capacity of a power source that powers the microcomputer 10, as shown in FIG. 7. In such cases, the supply voltage VDD1 may still be sufficient to operate the microcomputer 10 but let the output voltage VDD2 of the step-down circuit 13 decline, thereby lowering the level of the signal output from the serial output terminal 12. If the output signal T1 drops below the lower limit of the High-level signal to the peripheral circuit, the peripheral circuit can malfunction.
Conventional microcomputers of the above constitution are thus faced with a problem: a drop in the supply voltage to the microcomputer triggers a decrease in the signal level to a peripheral circuit, causing the latter to malfunction.